Microphone



. Patented; Feb. 25, 1930 UNITED} STATES JOHN HARRY DUNN RIDLEY,' or onnon, ENGLAND MICROPHONE 7 Application filed April 6, 1929, Serial No. 352,989, and in Great Britain March 28, 1928.

This invention relates to, and has for its object the provision of improvements in, microphones, and refers more particularly to microphones of the type comprising granul lar resistance material electrically connecting two electrodes and adapted to vary its resistance as a function of sound Waves directed to the microphone, the current flow between the electrodes being transverse to the incident 10 sound waves.

The invention comprises the arrangement according to -which said electrodes are of concave cross section and have their concave surfaces facing one another and in contact with said material. By-making the elec- 20 increased.

The invention further comprises the arrangement according to which said electrodes are; partially surrounded by means of coatings of rubber or like nonresonant material 25 so that the exposed surface is shielded from the sound waves directed to the microphone. The invention further comprises the arrangement according to which said electrodes are gripped directly. between surfaces of rubber or like nonresonant material in compression. v

In order that the invention may be the more clearly understood a microphone in accordance therewith will now be described, reference being made to the accompanying drawings wherein a 3 V Fig. 1 is a sectional side elevation of said microphone shown partly in section on line XX of Fig. 2.

phragm and the resistance granules removed.

F ig. 3 is an end view shown partly in sec- 4 tion on line YY of Fig. 1.

The numeral 1 designates ablock of insulating material'made of a number of laminations 11.

- A rectangular opening is cut through the upper lamination 1 and mounted within this is a. rectangular medial lamination l. This Fig. 2 is a plan of the same with the dia opening upon'the' succeeding lamination 1 lamination 1 is of a depth say threequarters of say-three times the depth of said recess a.

The electrodes 2 arelocated respectively in said troughs b; the granular resistance filling 3 fills theshallow recess a together with the unoccupied portion of said troughs b, and the whole recess including the troughs is covered by means of a diaphragm 4. This medial lamination or block '1 is of greatimportance because it forms a shallow recess below the diaphragm to be filled with granular resistance filling. It will be seen that the path between electrodes 2 is very long compared to the thickness of the layer of granular resistance material between the diaphragm 4 and lamination 16, thus greatly increasin'gthe sensitivity of the instrument without a corresponding increase of background noise due. to the current flow.

The electrodes 2 and channel section and are arranged in the troughs with their con cave surfaces facing one another. Their convex surfaces are covered by means of a layer 5 of rubber, or in other words each channel section electrode nests within achannel section rubber locating piece.

I The bottom of eachtrough b i. e; thetop surface of the lamination 1 1s recessed as shown for the reception of the respective locating piece 5, and these recesses andalso the lower channel walls of the locating pieces 5 and electrodes 2 extend endwise underneath. thelamination 1. Immediately above the extending ends'of said lower walls of said electrodes, said lamination 1 is recessed and rubber plugs 18 are let into the.

'recesses, the arrangement being such that when said lamination 1 is screweddown on to the lamination 1", said rubber plugs 18 are compressed against said extending ends so that said lower walls of said electrodes are gripped between rubber on both sides and the electrodes are accordingly held firmly in situ in a nonresonant mounting.

- 2 and locating pieces 5 extend from one end of the troughs b to the other, and the outer surfaces of the upper wall of the locating.

pieces are flush with the diaphragm 4.. The back of eachlocating piece lies flush against the outer wall of the respective trough b but a considerable clearance remains between the inner wall of the trough b and the channel edges both of the locating piece 5 and of the electrode 2. i

The diaphragm 4 iscovered and supported by means of an insulating fixing plate 6 having a central aperture 0 through which access to said diaphragm is afforded.

' The conducting leads 7 pass. to said electrodes 2 through the insulating block 1 from terminals 8 mounted on the opposite side thereof.

Said electrodes 2 are composed of a nonoxidizable material such. as gold plated brass or copper. The material of the diaphragm 4 may suitably be celluloid about one or two thousandths of an inch in thickness. The granular filling 3 consists of a 7 5% filling of not finer than 200 mesh tothe inch carbon granules, 20% filling of carbon powder of the finest grade and 5% filling of graphite powder of commercial quality and of finest grade. The insulating body which may suitably be of ebonite should be loaded by means of a heavy metal. The lamination 1 for example may be made of lead.

' By substituting medial laminations 1 of varying depths the degree of sensitivity and response of the instrument may be varied.

It will be appreciated that any resonance in the electrodes 2 is damped out by the surrounding rubber of the locating pieces 5. The sensitivity moreover is maintained but the background noise due to current flow is lessened by use of the above filling. ;The length of the path between the electrode gives an increase of sensitivity.

In front of the diaphragm 4 a piece of gauze 9' may be secured by being clamped between said insulating plate 6 and a similar metallic plate 10, said plates 6 and 10 being secured in position by means of screws 11 screwed into metal bushes 12. Holes 13, plugged by means of screws 14, are provided for giving access to the resistance filling. The numeral 15 designates eyelets whereby the microphone is adapted to be supported in situ. These eyelets are. carried by plates 16 secured to the block 1 by means of screws 17.

What I claim and desire to secure. by Letters Patent is 1. A microphone comprising a pair of parallel elongated electrodes having their adjacent surfaces concave, non-resonant material covering the remote surfaces of the electrodes, a thin layer of granular resistsulating material mounted on the upper face of said base of shorter length than said base,

,7 horizontal elongated electrodes having adjacent concave faces mounted on the upper face of said base on either side of said block and spaced therefrom, the height of said electrodes and said block being substantially equal, a diaphragm mounted above said block and electrodes, the plane of the diaphragm and block being substantially parallel and the distance between the diaphragm and block being less than the distance between the electrodes, and granular resistance mater ialfilling the space between the diaphragm, electrodes and the block.

3. A'microphone comprising a base of insulating material, a rectangular block of insulatin material mounted in the upper face of saidliase of shorter length than said base, horizontal elongated electrodes having adjacent concave faces mounted on the upper face of said base on either side of saidblock and spaced therefrom, the height of said electrodes and said block being substantially equal and the adjacent faces of said electrodes being concave, a diaphragm mounted above said block and electrodes, the plane of the diaphragm and block being parallel and the distance between the diaphragm and block being less than the distance between electrodes, and granular resistance material filling the space between the diaphragm, electrodes and the block.

4. A microphone comprising a pair of parallel elongated electrodes, having. their adJacent surfaces concave, granular resistance material electrically connecting'said electrodes, and a diaphragm adapted to compress said resistance material upon vibration, the granular resistance material comprising carbon anules larger than 200 mesh to the inch, finely divided carbon powder, and finely divided graphite powder.

5. A microphone comprising a pair of parallel elongated electrodes, having their adjacent surfaces concave, granular resistance material electrically connecting said electrodes, and a diaphragm adapted to compress said resistance material upon vibration, the granular resistance material comprising approximity 75% carbon granules larger than 200 mesh to the inch, 20% finely divided carbon powder, and 5% finely divided graphiso granular resistance material electrically con- I necting said electrodes and a diaphragm adapted to compress said layer upon vibration.

7. A microphone comprising a base of insulating material, a detachabl'y mounted rectaniular block of insulating material secured to t e u per face of said base and of shorter length t an said base, electrodes mounted on the 11 per face of the base and spaced from the b ock, a diaphragm mounted above said base and said electrodes, and granular resis'tance material filling the 's ace between the diaphragm electrodesandt e block. 7

In witness whereof I- aflix 111 Si ature.

JOHN HARRY DUNN R LEY. 

